Molten carbonate fuel cells convert chemical energy into electrical energy. Molten carbonate fuel cells are useful in that they deliver high quality reliable electrical power, are clean operating, and are relatively compact power generators. These features make the use of molten carbonate fuel cells attractive as power sources in urban areas, shipping vessels, or remote areas with limited access to power supplies.
Molten carbonate fuel cells are formed of an anode, a cathode, and an electrolytic layer sandwiched between the anode and cathode. The electrolyte includes alkali carbonate salts, alkaline-earth carbonate salts, molten alkali carbonate salts, or mixtures thereof that may be suspended in a porous, insulating, and chemically inert matrix. An oxidizable fuel gas, or a gas that may be reformed in the fuel cell to an oxidizable fuel gas, is fed to the anode. The oxidizable fuel gas fed to the anode is typically syngas—a mixture of oxidizable components, molecular hydrogen, carbon dioxide, and carbon monoxide. An oxidant-containing gas, typically air and carbon dioxide, may be fed to the cathode to provide the chemical reactants to produce carbonate anions. During operation of the fuel cell, the carbonate anions are constantly renewed.
The molten carbonate fuel cell is operated at a high temperature, typically from 550° C. to 700° C., to react oxygen in the oxidant-containing gas with carbon dioxide to produce carbonate anions. The carbonate anions cross the electrolyte to interact with hydrogen and/or carbon monoxide from the fuel gas at the anode. Electrical power is generated by the conversion of oxygen and carbon dioxide to carbonate ions at the cathode and the chemical reaction of the carbonate ions with hydrogen and/or carbon monoxide at the anode. The following reactions describe the electrical electrochemical reactions in the cell when no carbon monoxide is present:                Cathode charge transfer: CO2+0.5O2+2e−→CO3=        Anode charge transfer: CO3=+H2→H2O+CO2+2e− and        Overall reaction: H2+0.5O2→H2O        
If carbon monoxide is present in the fuel gas, the chemical reactions below describe the electrochemical reactions in the cell.                Cathode charge transfer: CO2+O2+4e−→2CO3=        Anode charge transfer: CO3−+H2→H2O+CO2+2e− and CO3=+CO→2CO2+2e−        Overall reaction: H2+CO+O2→H2O+CO2         
An electrical load or storage device may be connected between the anode and the cathode to allow electrical current to flow between the anode and cathode. The electrical current powers the electrical load or provides electrical power to the storage device.
Fuel gas is typically supplied to the anode by a steam reformer that reforms a low molecular weight hydrocarbon and steam into hydrogen and carbon oxides. Methane, for example, in natural gas, is a preferred low molecular weight hydrocarbon used to produce fuel gas for the fuel cell. Alternatively, the fuel cell anode may be designed to internally effect a steam reforming reaction on a low molecular weight hydrocarbon such as methane and steam supplied to the anode of the fuel cell.
Methane steam reforming provides a fuel gas containing hydrogen and carbon monoxide according to the following reaction: CH4+H2O⇄CO+3H2. Typically, the steam reforming reaction is conducted at temperatures effective to convert a substantial amount of methane and steam to hydrogen and carbon monoxide. Further hydrogen production may be effected in a steam reformer by conversion of steam and carbon monoxide to hydrogen and carbon dioxide by a water-gas shift reaction of: H2O+CO⇄CO2+H2.
In a conventionally operated steam reformer used to supply fuel gas to a molten carbonate fuel cell, however, little hydrogen is produced by the water-gas shift reaction since the steam reformer is operated at a temperature that energetically favors the production of carbon monoxide and hydrogen by the steam reforming reaction. Operating at such a temperature disfavors the production of carbon dioxide and hydrogen by the water-gas shift reaction.
Since carbon monoxide may be oxidized in the fuel cell to provide electrical energy while carbon dioxide cannot, conducting the reforming reaction at temperatures favoring the reformation of hydrocarbons and steam to hydrogen and carbon monoxide is typically accepted as a preferred method of providing fuel for the fuel cell. The fuel gas typically supplied to the anode by steam reforming, either externally or internally, therefore, contains hydrogen, carbon monoxide, and small amounts of carbon dioxide, unreacted methane, and water as steam.
Fuel gases containing non-hydrogen compounds such as carbon monoxide, however, are less efficient for producing electrical power in a molten carbonate fuel cell than more pure hydrogen fuel gas streams. At a given temperature, the electrical power that may be generated in a molten carbonate fuel cell increases with increasing hydrogen concentration. This is due to the electrochemical oxidation potential of molecular hydrogen relative to other compounds. For example, Watanabe et al. describe, in “Applicability of molten carbonate fuel cells to various fuels,” Journal of Power Sources, 2006, pp. 868-871, that a 10 kW molten carbonate fuel cell stack operated at 90% fuel utilization and a pressure of 0.49 MPa at a current density of 1500 A/m2, with a 50% molecular hydrogen and 50% water feed produces an electrical power density of 0.12 W/cm2 at 0.792 volts while a 50% carbon monoxide and 50% water feed at the same operating conditions produces an electrical power density of only 0.11 W/cm2 at 0.763 volts. Therefore, fuel gas streams containing significant amounts of non-hydrogen compounds are not as efficient in producing electrical power in a molten carbonate fuel cell as fuel gases containing mostly hydrogen.
Molten carbonate fuel cells, however, are typically operated commercially in a “hydrogen-lean” mode, where the conditions of the production of the fuel gas, for example, by steam reforming, are selected to limit the amount of hydrogen exiting the fuel cell in the fuel gas. This is done to balance the electrical energy potential of the hydrogen in the fuel gas with the potential energy (electrochemical+thermal) lost by hydrogen leaving the cell without being converted to electrical energy.
Certain measures have been taken to recapture the energy of the hydrogen exiting the fuel cell, however, these are significantly less energy efficient than if the hydrogen were electrochemically reacted in the fuel cell. For example, the anode exhaust produced from the electrochemically reacting fuel gas in the fuel cell has been combusted to drive a turbine expander to produce electricity. Doing so, however, is significantly less efficient than capturing the electrochemical potential of the hydrogen in the fuel cell since much of the thermal energy is lost rather than converted by the expander to electrical energy. Fuel gas exiting the fuel cell also has been combusted to provide thermal energy for a variety of heat exchange applications. Almost 50% of the thermal energy, however, is lost in such heat exchange applications after combustion. Hydrogen is a very expensive gas to use to fire a burner utilized in inefficient energy recovery systems and, therefore, conventionally, the amount of hydrogen used in the molten carbonate fuel cell is adjusted to utilize most of the hydrogen provided to the fuel cell to produce electrical power and minimize the amount of hydrogen exiting the fuel cell in the fuel cell exhaust.
Other measures have been taken to produce more hydrogen from the fuel gas that is present in the anode exhaust and/or recycle hydrogen in the anode gas by providing the fuel gas to post reformers and/or gas separation units. To recover the hydrogen and/or carbon dioxide, the fuel gas present in the anode is reformed in the post reformer to enrich the anode gas stream in hydrogen and/or subjected to a water-gas shift reaction to form hydrogen and carbon dioxide. Heat may be provided by the anode gas stream.
Heat for inducing the methane steam reforming reaction in a steam reformer and/or converting liquid fuel into feed for the steam reformer has also been provided by burners. Burners that combust an oxygen-containing gas with a fuel, typically a hydrocarbon fuel such as natural gas, may be used to provide the required heat to the steam reformer. Flameless combustion has also been utilized to provide the heat for driving the steam reforming reaction, where the flameless combustion is also driven by providing a hydrocarbon fuel and an oxidant to a flameless combustor in relative amounts that avoid inducing flammable combustion. These methods for providing the heat necessary to drive steam reforming reactions and/or water-gas shift reactions are relatively inefficient energetically since a significant amount of thermal energy provided by combustion is not captured and is lost.
The hydrogen and carbon dioxide in the reformed gas stream may be separated from the anode exhaust, for example, using pressure swing adsorption units and/or membrane separation units. The temperature of the anode exhaust is typically higher than the temperatures required by commercial hydrogen and/or carbon dioxide separation units. The stream may be cooled, for example, through a heat exchanger, however, thermal energy may be lost in the cooling process.
The separated hydrogen is fed to the anode portion of the fuel cell. Recycling the hydrogen to the anode may enrich the fuel gas entering the molten carbonate fuel cell with hydrogen. The separated carbon dioxide is fed to the cathode portion of the fuel cell. Recycling the carbon dioxide to the cathode may enrich the air entering the molten carbonate fuel cell with carbon dioxide.
The cell potential (V) of a molten carbonate fuel cell is given by the difference between the open circuit voltage (E) and the losses. For high temperature fuel cells, activation losses are very small and the cell potential may be obtained over the practical range of current densities by considering only ohmic losses. Thus cell potential V=E−iR, where V and E have units of volts or millivolts, i is the current density (mA/cm2) and R is the total Ohmic resistance (Ωcm2), combining electrolyte, cathode and anode together. The open circuit voltage is the dominant term in the cell potential. The total voltage (electromotive force) for a molten carbonate fuel cell can be expressed using the Nernst Equation, E=Eo+(RT/2F)ln(PH2PO20.5/PH2O)+(RT/2F)ln(PCO2c/PCO2a), where E is the standard cell potential, R is the universal gas constant of 8.314472 JK−1mol−1, T is the absolute temperature, and F is the Faraday constant of 9.64853399×104 C mol−1. As shown, the cell voltage of a molten carbonate fuel cell may be changed by varying the concentrations of carbon dioxide, hydrogen, and oxygen.
Certain measures have been taken to adjust the concentration of hydrogen, oxygen, and carbon dioxide provided to the fuel cell to maximize cell voltage. U.S. Pat. No. 7,097,925 (the '925 Patent) maximizes the denominator of the ratio
            P              H        ⁢                                  ⁢        2        ⁢                                  ⁢        O        ⁢                                  ⁢                  (          anode          )                      ·          P              CO        ⁢                                  ⁢        2        ⁢                                  ⁢                  (          anode          )                                P              H        ⁢                                  ⁢        2        ⁢                                  ⁢                  (          anode          )                      ·          P              O        ⁢                                  ⁢        2        ⁢                                  ⁢                  (          cathode          )                    0.5        ·          P              C        ⁢                                  ⁢        O        ⁢                                  ⁢        2        ⁢                                  ⁢                  (          cathode          )                    by enriching streams fed to the anode of a molten carbonate fuel cell with hydrogen while enriching streams fed to the cathode with oxygen and carbon dioxide. The enriched streams are provided from pressure swing adsorption units.
While the prior art is effective in providing hydrogen, oxygen, and carbon dioxide to the fuel cell at different concentrations, the process is relatively inefficient in producing the hydrogen, carbon dioxide, and oxygen streams. The process is also relatively thermally inefficient in production of the gases and thermal processes since the anode gas is cooled to remove the water prior to entering the pressure swing adsorption units. In addition, the reformers also do not convert a liquid hydrocarbon feedstock to a lower molecular weight feed for the steam reformer, and insufficient heat is likely provided from the fuel cell to do so.
Further improvement in the efficiency in operating molten carbonate fuel cell systems for producing electricity and enhancing power density of the molten carbonate fuel cell is desirable.